1. Technical Field
This invention relates to vacuum interrupters and, more particularly, to vacuum interrupters that provide a radial magnetic field in the contact region to encourage formation of a diffuse arc between the vacuum interrupter contacts and a surrounding metal vapor shield.
2. Description of the Prior Art
Vacuum interrupters are typically used, for instance, to reliably interrupt medium to high voltage ac currents of several thousands of amperes or more. They generally include a vacuum envelope enclosing a pair of facing contact electrodes, one of which is movable relative to the other from a closed circuit position to an open circuit position. Each contact is connected to a current-carrying stem, or terminal post, extending outside the vacuum envelope. Surrounding the contacts within the envelope is a tubular metal vapor condensing shield aligned concentrically with the contacts and terminal posts.
When the contacts are moved apart from the closed circuit position to the open position, an arc forms in the metal vapor evaporated from the contacts. The arcing continues between the contacts until the current is interrupted. Metal from the contacts that is vaporized by the arc condenses back onto the contacts and onto the vapor shield. The vapor shield thus serves to protect the insulating vacuum envelope from accumulating deposits of metals.
The designs of practical contact arrangements for commercial high-current vacuum interrupters have evolved over the past thirty years into two principal types, discussed in an article authored by this inventor. P. G. Slade, The Vacuum Interrupter Contact, IEEE Trans. on Components, Hybrids, and Mfg. Tech., Vol. CMHT-7, No. 1, p. 25-32, March 1984, herein included in this specification by reference. Each type of contact arrangement produces a magnetic field that helps to control the initially columnar arc and promote its transition to a diffuse mode before the current reaches zero in an a.c. circuit. In a first type of contact arrangement, an axial magnetic field generated by coils located behind the contacts forces the high-current arc to rapidly become diffuse and continuously distributed within the contact gap. In a second type of contact arrangement, using spiral-arm or slotted-cup contacts, a magnetic field self-generated by the current in the contacts is impressed perpendicular to the arc column in a direction which forces the arc to move rapidly around the circular periphery of the contact surface. A typical spiral-arm contact will have four arms each extending no more than 180.degree. around the contact.
During high-current arcing with present spiral-arm or slotted-cup contact designs and a thick Cu--Cr vapor shield, the arc has been observed to do three things. First, it has been observed that the Lorentz force drives the arc along the peripheral edge of the contacts. Second, the arc column can be perturbed by a slot and attach from one contact to the vapor shield and back to the other contact and move across both the contacts and the shield. Third, the arc can attach to the contact and shield in a stationary, but somewhat diffuse arc. The vapor shield becomes, in effect, a third electrode in the second and third modes.
The third form of the vacuum arc is normally beneficial to the successful interruption of a high-current arc. When the vacuum arc burns between the contacts and shield, the magnetic field is along the line of current flow. This weak radial field causes the arc to become more diffuse and hence to deliver a smaller thermal load upon the contacts and the shield, thereby reducing the tendency to burn through the contacts or the shield. Reduction of burn-through allows more compact interrupter design with higher interruption capacity.
There is therefore a need for a vacuum interrupter that provides a greater radial magnetic field than that provided by prior art designs in order to encourage the promotion of the third form of high-current arcing.